1. Field of the Invention
Exemplary embodiments of the present invention relate to inorganic silicate-based luminescent substances which can be used as radiation converters for converting higher-energy primary radiation, for example (ultraviolet) UV radiation or blue light. The higher-energy primary radiation is converted into a longer-wave visible radiation and can therefore be is employed in corresponding light-emitting arrangements, such as light emitting diodes (LEDs) emitting colored or white light.
2. Discussion of the Background
Europium-activated alkaline earth metal oxyorthosilicates such as Sr3SiO5:Eu have become known for use in LEDs emitting colored or white light. The element strontium in these compounds may also be completely or partly replaced by other alkaline earth metal ions.
Such a strontium silicate-based luminescent substance for long-wave LEDs emitting UV light is described in WO 2004/085570AI with the structure Sr3-xSiO5:Eu2+, where x=0<x≦1. The luminescent substance is said to have high luminescence efficiency. WO 2006/081803AI discloses a luminescent substance from the class consisting of oxyorthosilicates with the structure (Sr, Ba, Ca)3SiO5:Eu.
These known luminophores emit in the yellow to orange range of the visible spectrum and, with excitation by UV radiation or blue light, have the high luminescence efficiencies required for the relevant technical applications. Moreover, they display small full-widths at half maximum of the emissions spectra, which are advantageous for various applications. They also have low temperature extinction.
US 2006/261309A1 has disclosed luminescent mixtures which emit yellow light and have two silicate-based phases. These luminescent mixtures have an emission intensity in the wavelength range from 555 nm to 580 nm if they are excited by a radiation source having a wavelength range of 220 nm to 530 nm. The first phase includes a crystal structure (M1)2SiO4, and the second phase includes a crystal structure (M2)3SiO5. M1 and M2 are each selected from the group which consists of Sr, Ba, Mg, Ca and Zn. At least one of the phases of the mixture is comprises greater than or equal to 0.0001 percent by weight of Mg, and at least one of the phases is activated with divalent europium (Eu2+). In addition, at least one of the phases comprises a dopant D which is selected from the group which consists of F, Cl, Br, S and N. At least one of the dopant atoms are arranged at lattice sites of the oxygen atom of the silicate crystal host of the luminescent substance.
WO 2007/035026A1 describes a silicate luminescent substance for emitting UV light with long-wave excitation, which has a color coordinate of x=0.50 to 0.64 and y=0.38 to 0.51. This is represented by the formula (Sr1-x-y-zAxBanZny)3SiO5:Rez, where A is at least one alkaline earth metal which is selected from Ca and Mg. R denotes a rare earth metal and 0≦x≦0.5; 0<y≦0.5; 0<z<0.2; and 0<n<1. The luminescent substance is prepared from a stoichiometric mixture of strontium, barium, zinc, and silicon dioxide as matrix components and rare earth metal as an active substance component. The resulting mixture is dried at 100 to 150° C. for preparation of the luminescent substance. This is followed by heat treatment of the luminescent substance at 800 to 1500° C. under a mixed gas atmosphere comprising nitrogen and hydrogen. 0.001 to 0.5 mol of alkaline earth metal and 0.001 to 0.5 mol of zinc are then added to the matrix component per 1 mol of strontium.
The disadvantage of these known luminescent substances is that they may have a relatively short lifespan under conditions of use. This is due in particular to the high moisture sensitivity of the europium-doped alkaline earth metal oxyorthosilicates. Such a substantial disadvantage may limit the technical applicability of the above luminophores.